The present invention relates generally to electrically programmable, phase-change memory elements and more particularly to a method for data storage using only the amorphous phase of such elements.
Programmable phase-change memory elements formed from materials that can be programmed to exhibit at least two detectably distinct electrical resistivities are known in the art. Phase-change materials may be programmed between a first structural phase where the material is generally more amorphous and a second structural phase where the material is generally more crystalline. The term amorphous as used herein, refers to a condition that is relatively structurally less ordered or more disordered than a single crystal and has a detectable characteristic, such as high electrical resistivity. The term crystalline, as used herein, refers to a condition that is relatively structurally more ordered than amorphous and has lower electrical resistivity than the same material has in the amorphous phase. Since memory elements made with a phase-change material can be programmed to a high resistance state or a low resistance state by changing the phase of the material, one phase can be used to store a logic 0 data bit, for example, while the other is used to store a logic 1 data bit.
A single pulse of energy referred to as a set pulse can be used to transform a volume of phase-change material from the high resistance, amorphous phase, to the low resistance, crystalline phase. Similarly, a single pulse of energy referred to as a reset pulse can be used to transform the volume of phase-change material from the crystalline phase to the amorphous phase. Each phase is non-volatile, i.e., stable, and has characteristic differences that are measurable, such as the change in resistance previously noted.
Electrical resistivity, however, is only one property that changes with a set or a reset of the phase-change material. For example, optical reflectivity also changes with the phase of the material. These changes result because the amorphous-to-crystalline transition is accompanied by discontinuous changes in the volume, density, thermal expansion co-efficient and other material parameters of the phase-change material. Due to these discontinuous changes in the phase-change material, operating the memory device in a phase-change mode is prone to failures. For example, one potential structural failure resulting from the discontinuous changes of the phase-change material is delamination of the phase-change material from the contacts of a memory device, particularly when operating at high frequencies and with high cycling. These types of problems are typically solved by thermal engineering of the structure of the memory element in an effort to minimize stress during operation. Another design solution is selecting suitable contacts to the phase-change material. Both of these solutions require careful engineering of the boundary conditions and interfaces to be manufactured into the memory element.
According to the present invention, a data storage capability is provided using memory elements of phase-change material that operate entirely within one phase, thus avoiding the problems associated with discontinuous changes. Performing only microscopic and gradual changes within a single phase minimizes structural failures such as delamination, without additional engineering of the memory element structure. By operating in the amorphous, or reset, phase, another memory element failure, failure to set (i.e., convert to the crystalline phase) upon receipt of a set pulse, can also be prevented. In addition, the amorphous phase requires low programming energy compared to the crystalline phase.
The present invention, therefore, is a method of data storage using a phase-change memory element operating in an amorphous phase. The memory element has a threshold voltage variable with a programmed resistance of the memory element. The threshold voltage is where the phase-change material starts exhibiting negative conductivity, that is, the phase-change material moves from the stable amorphous phase to an unstable electrical region where current increases but voltage decreases. The method includes a step of applying a voltage potential across a memory element programmed to one resistance state of a plurality of detectably distinct resistance states. The voltage potential is a discriminating voltage that is greater than or less than the threshold voltage of the programmed memory element. The method also includes the steps of preventing a current higher than a limiting current from flowing across the memory element if the discriminating voltage is greater than the threshold voltage and determining the resistance state of the memory element using a level of current flowing across the memory element.
Another aspect of the present invention is a method of operating a phase-change memory element, the memory element including a phase-change material, the method comprising the step of: programming the phase-change material between at least a first resistance state and a second resistance state without making an amorphous to crystalline phase transition. Preferably, the first resistance state corresponds to a first modification of an amorphous phase and the second resistance state corresponds to a second modification of the amorphous phase.
Other variations of the method of the present invention are contemplated and are described in detail herein.